This invention relates to control of devices located at the high-voltage terminal of an electrostatic accelerator such as a tandem Van de Graaff accelerator.
One of the features of the tandem Van de Graaff high-voltage electrostatic accelerator is a stripping terminal that is located at the high-voltage section at the center of the machine. In past applications this has quite frequently been a gas stripper that changes the ionization state of accelerated particles through collisions with the molecules of a gas. A number of advantages can be achieved with the use of foil strippers at the high-voltage terminal and this is especially true when accelerating heavy ions. One system that has been developed for the stripping of heavy ions is described in U.S. Pat. No. 3,806,749, "Method and Means of Effecting Charge Exchange in Particle Beams." That patent describes the use of a heated carbon foil as a stripping substance for placement in beams of particles such as nickel ions. It was disclosed in that patent that heated carbon foils could be used successfully for stripping for periods of the order of hours before it became necessary to change the foils. The necessity of controlling the heat and of operating a foil-changing mechanism at the high-voltage terminal subject to remote control from an operator at electrical ground who is removed from the radiation danger areas of the machine presents a technical challenge. The high-voltage terminal is typically at a voltage of the order of 6 to 12 million volts from electrical ground. It is common to maintain such a voltage by filling the interior of the accelerator with sulfur hexafluoride gas under a pressure of several atmospheres. Optimum use of the accelerator normally requires that the terminal be operated at voltages that are as high as the components can maintain without arcing or excessive leakage and the interior of the accelerator is likely to be very noisy electrically as a result of stray discharges.
One system that has been applied to control elements at the high voltage terminal uses a light pipe as the medium of communication between equipment at electrical ground and equipment at the high voltage terminal. The light pipe is capable of withstanding high voltages in the lengths that are normally necessary to reach the high voltage terminal and it passes physically through the atmosphere within the accelerator. However, an insulator between several million volts and ground requires the usual precautions to equalize voltage drops along its length. This is normally a string of electrical resistors in series, with each one connected at one end to the structure to be equalized so as to provide equalized voltage drops at spaces along the light pipe. This is a costly item and introduces an extra loading element to be supplied by the charging belt or chain of the accelerator. It also offers another element that can initiate an arc to threaten the continued operation of the accelerator.
It is an object of the present invention to provide a better way of controlling elements at the high-voltage terminal of an electrostatic accelerator.
It is a further object of the present invention to provide a means of controlling elements at the high-voltage terminal of a tandem electrostatic accelerator without making a direct physical connection between the terminal and electrical ground.
Other objects will become apparent in the course of a detailed description of the invention.